1. Field of the Invention
The present invention relates to a diarylethene compound, a photochromic diarylethene type copolymer and a method for the production of the same. The photochromic diarylethene type copolymer according to the present invention exhibits excellent adhesive properties and processabilities and fast photo-induced color change and thus is useful in applications such as optical recording media, photochromic windows, display elements, plastic mirrors, photochromic filters, photo switches, photosensitive drums, recording elements, solar batteries, lenses, cosmetics, fibers or optical parts.
2. Background of the Invention
The diarylethene type photochromic compounds have been known to be stable photochromic compounds which do not exhibit any heat-induced color change after being exposed to radiation of light since they were synthesized in 1985 (Japanese Patent Laid-Open Publication No. 86-263935 A2, J. Org. Chem., 1991, 49, 373). In accordance with the synthesis of diarylethene type photochromic compounds, various types of derivatives have been synthesized and suggested for use in applications such as reversible optical disks, erasable optical disks, photo switches for optical integrated elements, organic photosensitizers and photo electrodes (Japanese Patent Laid-Open Publication Nos. 91-261782 A2, 91-261781 A2, 91-261762 A2, 92-178383 A2, 92-178382 A2, 94-199846 A2, 91-135977 A2, 93-59025 A2, 93-169820 A2, 93-11406 A2, 93-301873 A2, 94-267071 A2, 95-72567 A2, 96-69083 A2, 96-245579 A2, 97-61647 A2, 97-80681 A2; Takeshita, M; Uchida, K.; Irie, M. Chem. Commun, 1996, 1807-1808).
Said diarylethene type compounds refer to compounds which, when exposed to the radiation of ultraviolet rays, change color and then return to their original color when exposed to radiation of a different wavelength. Various types of photochromic compounds have been suggested for use in applications in which a color change or darkening is induced by light (Japanese Patent Laid-Open Publication Nos. 95-89954 A2, 97-71585 A2, 97-77743 A2, 97-77767 A2 and 93-301873 A2).
Among them, fluorine-substituted diarylethenes or maleic anhydride-substituted diarylethenes are known to show high thermal stability, fast photo-induced color change, and accordingly methods for the production of optical recording film, in which the compounds are dissolved in a polymer resin and then coated by means of a solution coating method, were reported (J. Org. Chem., 1991, 49, 373).
However, when low molecular weight diarylethene compounds are introduced into a polymer resin, the compatibility of the compounds with a resin is insufficient, thereby resulting in the aggregation of the photochromic diarylethene compounds in the polymer medium to cause a phase separation. For this reason, the reliability and storage stability of the optical recording became insufficient to be used for long term use. In particular, when the low molecular weight diarylethene compounds are used in any applications related to photonic devices utilizing refractive index change by the reversible photochromic property, such phase separation problem of the low molecular weight diarylethenes causes poor reproductivity in their photochromic properties and refractive index change and errors in the signal processing and interpreting the signals.
In order to solve these problems, polymers having, as a branched chain, diarylethenes bound polymethacrylate system, have been developed (Japanese Patent Laid-Open Publication No. 06-240242 A2). However, such polymers have drawbacks in that (1) the synthesis of the diarylethene methacrylate monomer is complex due to its low temperature reaction, (2) the mechanical properties of thus prepared polymers are poor and (3) the adhesive property of thus prepared polymers on a glass plate or plastic plates such as PC, PMMA and PE is insufficient.
In addition, since such polymers are substituted with alkylenes which contain hydrogens or fluorines between the diarylethene and the methacryl groups, the polar excited state structure, formed when the diarylethene group is excited by the exposure to radiation of light, cannot be stabilized. Therefore, such polymer restricts the use of the polymers in applications such as a photochromic thin film utilizing the polarity of the excited state such as the rapid change in their refractive index within extremely short time.
The present inventors have made extensive studies in order to overcome the aforementioned problems caused in the preparation of the diarylethene type polymers and to develop a diarylethene type polymer which can be easily synthesized and isolated and which is also connected by a polar group such as a carboxyl group and which possesses excellent photochromic properties and mechanical properties and finally is capable of solving the problems associated with the phase separation or the formation of aggregate. As a result, the present inventors have now found that a photochromic polymer of the formula (4), which possesses excellent mechanical properties and processabilities as well as exhibiting fast photo-induced color change, can be prepared by polymerizing a diarylethene monomer of the formula (1) wherein benzene ring is substituted with Wxe2x80x94R2OC(xe2x95x90O)xe2x80x94 (wherein W is methacryl or acryl group), in the presence of an initiator, with at least one selected from the group consisting of a styrene type monomer of the formula (2) below and a compound of the formula (3) below. The present inventors have also found that thus prepared polymer may be dissolved in an organic solvent to form a composition, which may then be used to obtain a photochromic thin film. The present invention is attained on the basis of these findings.